One-pot thioetherification of aryl halides with thiourea and benzyl bromide in water catalyzed by Cu-grafted furfural imine-functionalized mesoporous SBA-15

Article abstract of DOI:10.1039/c2cc32676k

Surface functionalization of SBA-15 followed by its reaction with Cu(OAc)₂ has been carried out to develop a new Cu-grafted functionalized mesoporous material, which catalyzes one-pot three component coupling of different aryl halides with thiourea and benzyl bromide in aqueous medium to produce aryl thioethers in very good yields (80-88%).

Full text of DOI:10.1039/c2cc32676k

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COMMUNICATION
One-pot thioetherification of aryl halides with thiourea and benzyl
bromide in water catalyzed by Cu-grafted furfural imine-functionalized
mesoporous SBA-15w
a
a
a
b
b
John Mondal, Arindam Modak, Arghya Dutta, Sohini Basu, Shambhu Nath Jha,
b
a
Dibyendu Bhattacharyya and Asim Bhaumik*
Received 8th February 2012, Accepted 26th June 2012
DOI: 10.1039/c2cc32676k
Surface functionalization of SBA-15 followed by its reaction
with Cu(OAc) has been carried out to develop a new Cu-grafted
To eliminate these problems, new catalytic methods under
11
12
13
2
solvent-free conditions, using ionic liquids and water as the
reaction medium have been developed. Being a cheap, abundant,
non-toxic, non-flammable and relatively green solvent, replacement
of the organic solvents by water becomes an essential achievement
functionalized mesoporous material, which catalyzes one-pot
three component coupling of different aryl halides with thiourea
and benzyl bromide in aqueous medium to produce aryl thioethers
in very good yields (80–88%).
14
to address the industrial and environmental concerns.
Two novel protocols for C–S bond forming reactions, including
one-pot Michael addition reactions via an odourless process
involving in situ generation of S-alkylisothiouronium salts in
The construction of carbon–sulphur bonds represents an
indispensable tool for the synthesis of many target molecules
that have significant pharmaceutical impact. These compounds
are also used as molecular precursors for the design of new
1
5
water and one-pot thioetherification of aryl halides using
1
6
thiourea (free from the foul smell of thiols), have been
developed in recent years. But these newly developed protocols
are carried out in the presence of metal complexes as a
homogeneous catalyst, which has disadvantages of difficult
product separation from reaction mixture, recovery of catalyst
and problems associated with the recycling of the catalyst. One
of the simplest approaches is to immobilize the homogeneous
catalyst at the surface of an insoluble high surface area solid
1
functionalized materials. A number of drugs, which are applied
for the treatment of Alzheimer’s disease, Parkinson’s disease
and diabetes, as well as immune and inflammatory diseases,
2
carry aryl sulfide moieties at their backbone unit. In recent
years, a great deal of attention has been paid to the new, green
and efficient synthetic protocols for C–S bond construction,
3
especially under eco-friendly and safe reaction conditions.
4
1
7
Cross-coupling reactions mediated by transition metal catalysts
support. Functionalized silica materials containing surface
donor sites can bind the metal cations at the surface of the
catalyst strongly and, as a result, the possibility of the leaching
of active metal sites from the catalyst surface would be low
during the liquid phase reaction. Organically functionalized
mesoporous materials have gained increasing attention as
become a valuable tool in organic synthesis and material science
5
for the generation of new carbon–heteroatom bonds. Preparation
of thioethers via aryl sulfur coupling reaction of aryl halides and
thiols has been conducted in the presence of various catalysts, such
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7
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9
as Pd, Cu, Ni, Co and Fe salts. The problems traditionally
associated with these C–S coupling reactions include direct use of
volatile and foul-smelling thiols, which leads to environmental and
safety problems and limits the use of this method for large scale
operation. Moreover, these C–S coupling reactions are mostly
carried out in the presence of expensive, toxic, flammable
organic solvents and their disposal becomes a serious problem
1
8
19
20
21
adsorbents, catalysts, gas storage, conducting materials
etc. due to their high surface area and tunability of the surface
functionality. Metal sites are grafted inside the pores, thus
becoming an alternative inexpensive, non-air sensitive, recycl-
able and easily separable heterogeneous catalyst, which could
address industrial and environmental concerns.
1
0
for the chemical industry. Thus, designing an efficient and
environmentally friendly catalytic process for C–S coupling
reactions is highly desirable.
In this communication, we first report the synthesis of a new
furfural imine-functionalized mesoporous SBA-15 catalyst
grafted with Cu(II) (Scheme 1) and its excellent catalytic
activity in an efficient, one-pot, odourless process for
thioetherification reaction of aryl halides with benzyl bromide
a
Department of Materials Science, Indian Association for the
Cultivation of Science, Jadavpur, Kolkata-700 032, India.
E-mail: msab@iacs.res.in; Fax: +91 33 2473 2805;
Tel: +91 33 2473 4971
Applied Spectroscopy Division, Bhabha Atomic Research Centre,
Mumbai-400085, India
2 3
and thiourea in aqueous medium in the presence of K CO
base at 100 1C. This heterogeneous catalyst was developed via
surface functionalization with 3-aminopropyl-triethoxysilane,
b
followed by Schiff-base condensation of the surface –NH
2
w Electronic supplementary information (ESI) available: XRD, EXAFS,
groups with furfural. Cu(OAc) in absolute ethanol is allowed
2
N
2
sorption, MAS NMR, EPR, FT IR, UV of Cu-F-SBA-15, recycling of
catalyst, H and C NMR of thioethers. See DOI: 10.1039/c2cc32676k
1
13
to react with the Schiff-base anchored mesoporous SBA-15 to
8
000 Chem. Commun., 2012, 48, 8000–8002
This journal is c The Royal Society of Chemistry 2012

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Scheme 1 The synthesis of Cu-anchored mesoporous SBA-15
catalyst (Cu-F-SBA-15).
produce novel Cu(II) grafted mesoporous SBA-15 catalyst
Cu-F-SBA-15 (Scheme 1).
The small angle X-ray powder diffraction pattern for 3-amino-
propyl functionalized SBA-15 (A) (Fig. S1a, ESIw) shows three
characteristic diffraction peaks in the 0.9 to 2.0 degrees of 2y and
these can be attributed to the 100 (strong), 110 (weak) and
Fig. 1 HR TEM images of fresh (A) and reused (B) catalyst; EPR (C)
and EXAFS (D) of Cu-F-SBA-15 catalyst: fresh (a) and reused (b).
2
00 (weak) reflections, respectively, corresponding to the
D-hexagonal mesophase. When this 3-aminopropyl-functionalized
SBA-15 (A) was subjected to Schiff-base condensation (B) and
subsequent reaction with Cu(OAc) (C), then a considerable
which is strong evidence for the presence of discrete CuON units.
3
2
These g and A values resemble those for Cu(II) ions in the distorted
2
4
square planar symmetry. For the reused catalyst (Fig. 1Cb) the
2
four splitting patterns and the g>, g and A values remain
J
J
decrease in the intensities of the peaks are observed (Fig. S1b
and c, ESIw), however a single intense peak is present in the
both samples, suggesting the preservation of the mesophase.
This decrease in the peak intensities on grafting of Cu at the
unchanged, suggesting that the geometry and the environment
of the catalyst remain unaltered after reuse. The EPR spectrum of
Cu(OAc) on the other hand showed dimeric Cu species with a
2
much shorter Cu–Cu distance (Fig. S9, ESIw). Furthermore, the
2
2
surface could be attributed to the lowering of local order. The
wide angle XRD pattern of Cu-F-SBA-15 suggested a new
orthorhombic phase of the pore wall and this has been retained
after catalytic recycling (Fig. S2–S3, Table S1, ESIw).
EXAFS spectra of Cu-F-SBA-15 (Fig. 1D) of the fresh and the
used catalyst show very small pre-edge peaks due to 1s-3d
transition at 8977.9 eV, corresponding to the oxidation state of
25
Cu as B+2. Their edge structures are very similar to each other.
This implies that the local structures around the Cu atom in the
reused catalyst remain unchanged after the catalytic reaction.
The catalyst environment and the co-ordination number of Cu
in the fresh and the reused catalyst are obtained from the radial
distribution curve (Fig. S10 and Table S2, ESIw).
N sorption analysis suggested Cu-F-SBA-15 (Fig. S4, ESIw)
2
2
has a BET surface area and pore volume of 117 m g and
À1
À1
0
.126 ccg , respectively. SEM analysis suggested that the
material has a rod-shaped particle morphology (Fig. S5, ESIw).
Furthermore, FTIR spectra indicate the coordination of Cu
with the imine CQN bond and bridging acetate ions as shown
for species C in Scheme 1 (Fig. S6, ESIw). UV-vis spectra
One-pot thioetherification (Scheme 2) of different aryl halides
using thiourea and benzyl bromide in aqueous medium at 100 1C
over Cu-F-SBA-15 resulted in different aryl alkyl thioethers
(Table 1) in good yields. All of the products are characterized
(
Fig. S7, ESIw) suggested the charge transfer and d–d transitions of
+2 13 29
Cu . Furthermore, solid state C and Si MAS NMR results
suggested the presence of organic species and Si environments
in Cu-F-SBA-15 (Fig. S8, ESIw). The HR TEM image of
Cu-F-SBA-15 (Fig. 1A) clearly suggests a uniform honeycomb-
like hexagonal array of ordered pores (dimension ca. 4.0–4.6 nm).
The FFT diffractogram (inset) suggested 2D-hexagonal pore
1
13
by H and C NMR, respectively (ESIw, S12). The Cu content of
the fresh catalyst was 0.986 mmol g , as measured by ICP-AES
À1
analysis. The reaction takes 10–12 h for completion for
all bromo- and chloro-arenes and the respective turnover
frequencies (TOFs) are moderately high at ca 85–112
(Table 1). In order to check the heterogeneous nature of the
catalyst, a hot filtration test, leaching test and three phase test
were performed (ESIw, S1). After separation from the reaction
mixture, the recovered catalyst was successively washed with
copious amounts of water to remove excess base, followed by ethyl
2
3
channels. Furthermore, in this TEM image the black colour
spots could be attributed to the presence of grafted Cu-sites
formed due to the coordination of imine-N and furfural-O atom
with Cu(II) at the surface. The TEM image of used Cu-F-SBA-15
catalyst suggested the retention of the hexagonal pores after the
reaction (Fig. 1B). EPR spectra for the Cu-F-SBA-15 (Fig. 1Ca)
showed four splitting features (m
I
= À3/2, À1/2, +1/2, +3/2)
in the low-field region for the parallel component due to
the hyperfine interaction between the unpaired electron and
the nuclear spin of copper (I = 3/2), whereas the signal for the
perpendicular component (g> = 2.06) remains un-resolved.
2
+
These characteristics are typical for isolated Cu cations in
2
axial symmetry. Also, the EPR spectrum of Cu-F-SBA-15
4
Scheme 2 Cu-anchored mesoporous SBA-15 catalyst mediated one-pot
revealed the following parameters: g = 2.26 and A = 172G,
J
thioetherification of aryl halides.
J
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Chem. Commun., 2012, 48, 8000–8002 8001

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Table 1 Thioetherification of different aryl halides (Br, Cl) in aqueous
medium over Cu-F-SBA-15 using thiourea
suggests the future potential application of this catalytic system
for the synthesis of different unsymmetrical aryl alkyl thioethers.
Time
Notes and references
À1
Entry Aryl Halides (h)
Product
Yield(%) TOF(h
)
1
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80
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1
Cu(OAc)
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ÁH
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O, SBA-15-supported Cu nanocatalyst, pure
2
SBA-15 and furfural-SBA-15 (Table S3, ESIw), showed very poor
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reaction in aqueous medium for the synthesis of aryl thioethers.
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over a furfural imine-functionalized Cu-grafted mesoporous
SBA-15 heterogeneous catalyst. This procedure is free from
foul-smelling thiols and work-up becomes easy, practical and
eco-compatible, diminishing environmental concerns. Commercially
available aryl halides make this procedure much easier than using
corresponding thiols via in situ generation of thiolate ions. The
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2
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002 Chem. Commun., 2012, 48, 8000–8002
This journal is c The Royal Society of Chemistry 2012